Change the code when the `math/bits` dust settles

[Yawning]

Note: Low priority since the one architecture that actually matters to the vast majority of people, has appropriate intrinsics, so the unsafe behavior of the fallback code never comes into play.

When the Go math/bits issues get resolved (one way or another, though I'm of the opinion that the functions should be constant time, with explicitly vartime alternatives provided separately), change the implementation to enable the 64 bit code path on more architectures.

See:

• math/bits: Add, Mul and Sub are not constant time golang/go#31229
• math/bits: guarantee Add, Sub, Mul, RotateLeft, ReverseBytes to be constant-time operations golang/go#31267